Moisture sensor

ABSTRACT

A sensor for indicating moisture, comprising a unit whose conductivity is dependent on the current moisture exposure and an electric circuit, said unit being included as a component in said electric circuit and said electric circuit being activable by applying an electric field and/or a magnetic field over the same to generate a measurable signal, which is dependent on the total resistance of the circuit. The unit comprises an enzyme together with a substrate, which enzyme is activated when moisture exposure occurs to catalyse a reaction with the substrate that is dependent on the degree of moisture exposure, the substrate reaction product increasing the conductivity of the unit. The invention also relates to the use of such a sensor and a method of using a sensor to indicate moisture.

FIELD OF THE INVENTION

[0001] The present invention relates to a sensor. More specifically, theinvention relates to a sensor for indicating moisture according to thepreamble to claim 1.

BACKGROUND ART

[0002] Moisture detection is important in a number of differentsituations. Moisture is undesirable whether it appears in houses,textiles, packaging materials, electronic appliances, chemicals, dryfoodstuffs, etc. Our industrialized society produces increasinglyrefined products, many of which are highly sensitive to moisture.

[0003] A plurality of prior art devices for detecting moisture areavailable on the market. WO0016081 discloses an electronic sensorcomprising a resonant circuit. The resonant circuit comprises, interalia, electrically conductive particles and a material that is capableof swelling in the presence of moisture. When the material swells theconductive particles are drawn apart and the conductivity of themoisture sensitive material decreases and the electrical resistance ofthe material increases. The electronic sensor is read by means of adetector, which generates an electromagnetic field.

[0004] U.S. Pat. No. 5,463,377 discloses a sensor comprising a circuitwith two electrodes connected to a switch. The switch closes in thepresence of an electrically conductive liquid between the electrodes.The switch may also close by a change in conductivity of a materialbetween the electrodes or by expansion of a liquid absorber that pushesthe electrodes together.

[0005] U.S. Pat. No. 6,097,297 discloses a sensor that is in permanentcontact with the detector. The sensor comprises two conductive electrodestrips which have an absorbing material located between them. When aliquid, preferably urine, is absorbed, the resistance between theelectrodes changes, which sends a signal to the detector.

[0006] GB 2,183,344 discloses, inter alia, a sensor for detectingmoisture. The sensor comprises a ceramic substrate with a tin oxidefilm, the substrate being connected on both sides to platinumelectrodes. In the presence of moisture, the electrical conductivityacross the substrate changes, which is detected by means of theelectrodes.

[0007] U.S. Pat. No. 4,327,731 discloses a system for indicatingmoisture in a diaper comprising an enzyme and a substrate as well as achromogen. In the presence of moisture, the substrate is dissolved,which catalyses reactions with the enzyme and the chromogen resulting ina coloured pigment being produced.

[0008] WO 9956690 concerns an incontinence pad based on theabove-mentioned technique disclosed in U.S. Pat. No. 4,327,731. The padis divided into zones which have varying urine absorption capacity andwhich undergo colour changes when exposed to moisture, therebypermitting a quantitative estimate of urine incontinence to beindicated.

[0009] In addition, there are sensors comprising an enzyme and asubstrate that are adapted for other fields of use, such as the sensordisclosed in WO 0125472, which is an activable time-temperatureindicator for determining and/or controlling the durability status of aproduct. The enzyme in this sensor is activated at a time 0 by beingbrought into contact with the substrate, for example in connection withthe packaging of the finished product.

SUMMARY OF THE INVENTION

[0010] One object of the present invention is to provide an improvedsensor, which, preferably, can be used to indicate the quantity ofmoisture to which the sensor is exposed.

[0011] A second object of the present invention is to provide a sensorthat is capable of indicating the current moisture status in an area ora product.

[0012] A third object is to provide a moisture sensor that allows rapid,rational and objective reading.

[0013] A fourth object of the present invention is to provide a way ofusing a sensor for indicating moisture.

[0014] A fifth object of the present invention is to provide a method ofindicating moisture by means of a sensor.

[0015] To achieve the first three objects, a sensor is provided inaccordance with claim 1. Preferred embodiments are stated in claims2-12.

[0016] To achieve the additional objects, a way of using a sensoraccording to claims 13, 14 and 15 and a method of measuring moisture bymeans of a sensor according to claim 16 are provided.

[0017] More specifically, according to the present invention a sensorfor indicating moisture is defined, comprising a unit whose conductivityis dependent on the current moisture exposure, and an electric circuit,said unit being included as a component in said electric circuit andsaid electric circuit being activable by applying an electric fieldand/or a magnetic field over the same to generate a measurable signal,which is dependent on the total resistance of the circuit. The unitcomprises an enzyme together with a substrate, which enzyme is activatedby moisture exposure to catalyse a reaction with the substrate that isdependent on the degree of moisture exposure, the substrate reactionproduct increasing the conductivity of the unit.

[0018] The catalytic activity of the enzyme is dependent on thesurrounding moisture. The enzyme activity results in the substrate beingtransformed into reaction products. Owing to the enzyme activity thequantity of reaction products increases as does also the conductivity ofthe unit as a function of the time and temperature at which moistureexposure occurs. The conductivity of the reaction products affects thetotal resistance of the circuit. When activating the electric circuit, asignal is generated that is dependent on the total resistance of thecircuit, which in turn is dependent on the conductivity of the reactionproducts. The signal thus also indicates, besides the conductivity ofthe reaction products, the quantity of moisture to which the sensor hasbeen exposed. The electric circuit is activated by applying an electricfield and/or a magnetic field over the same. The signal that isgenerated when activating the circuit is preferably a measurable currentinduced in the circuit.

[0019] A preferred embodiment of the present invention will be describedbelow for the purpose of exemplification with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0020]FIG. 1 is a schematic view of a sensor according to a preferredembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0021] Reference is now made to FIG. 1, which schematically shows asensor according to a preferred embodiment of the present invention.

[0022] The sensor comprises a unit 1, including an enzyme 1 a and asubstrate 1 b as well as an electric circuit 2, which is affected bysaid unit 1 and adapted to generate a signal when activated.

[0023] The unit 1 is preferably made of an absorbent material with goodabsorbing capacity, such as filter paper. The absorbent material ispreferably arranged with a layer of a barrier material which allowsmoisture transport in one direction, i.e. which allows moisturetransport from the surroundings but prevents evaporation from theabsorbent material.

[0024] The enzymes 1 a that are relevant to the present invention arethe ones that cause a change in the conductivity of the substrate.Enzymes which catalyse a reaction, in which as many ions as possibleform, are particularly preferred. The reason for this is that therelative change in conductivity is thus maximised. Enzymes having thisproperty can be found in the following enzyme classes: Transferases,Hydrolases and Lyases.

[0025] A particularly preferred enzyme 1 a is Urease, from the enzymeclass Hydrolases, which by means of water catalyses a hydrolyse of Ureato form carbon dioxide and ammonia.

[0026] Both the enzyme 1 a and its substrate 1 b according to thepresent invention are thus well-defined and preferably immobilized onthe absorbent material.

[0027] In one embodiment, the enzyme and the substrate are arranged inlayers on the absorbent material.

[0028] When exposed to moisture, the enzyme 1 a in the unit 1 isactivated catalysing a reaction with the substrate 1 b, whose reactionproduct will increase the conductivity of the unit 1 from its initialzero value when the unit was dry. The higher the moisture content in theunit 1 the higher the enzyme activity, which results in an increasingproportion of reaction products and thus higher conductivity of the unit1. Owing to the enzyme activity the quantity of reaction productsincreases as does the conductivity of the unit as a function of the timeand temperature at which moisture exposure occurs. In applications wherethe temperature is essentially constant, it is therefore possible toquantify the amount of moisture to which the sensor has been exposed.

[0029] The substrate 1 b may also comprise a buffer solution in order tocounteract any changes in pH of the substrate 1 b. Extreme pH valuescapable of deactivating the enzyme 1 a are thus avoided.

[0030] The unit 1 is included as a component in an electric circuit 2.The electric circuit 2, except said unit 1, is preferably insulatedagainst the surroundings, for example by means of a plastic material, toprevent moisture from affecting the circuit, which would lead toshort-circuiting thereof.

[0031] Preferably, the circuit 2 is an oscillation circuit and comprisesa coil 3, a capacitor 4 and a resistor 5, which are connected in seriesin a closed circuit. The unit 1 is connected in series with the othercomponents 3, 4 and 5. To activate the circuit 2, a magnetic field M isapplied over the coil 3 to induce a current in the circuit 2.

[0032] As shown in FIG. 1, the unit 1 is connected to the electriccircuit 2 via two electrodes 2 a, 2 b. In order to close the circuit 2,the substrate 1 b must thus exhibit such electric conductibility that acurrent can flow between the electrodes 2 a, 2 b, the current intensitybeing stronger the higher the conductivity. To adapt the sensor, thedistance between the electrodes 2 a, 2 b can be modified. Moreover, itis possible to modify the cross-sectional area of the unit 1 as well asthe concentration of both the enzyme 1 a and the substrate 1 b. Finally,various buffer concentrations can also be used.

[0033] The electric circuit 2 is activable by applying an electric fieldand/or a magnetic field M over the same to generate a measurable signal,which is dependent on the total resistance of the circuit 2. Said signalis preferably a measurable current, which is generated in the circuit 2.

[0034] Owing to its design the inventive sensor can be mass produced asa disposable sensor at a low cost. The sensor is preferably arranged ona label to be applied to or in the product or product container. Thelabel comprises, for instance, a carrier, to the underside of which anadhesive layer is applied. The sensor forms a layer, which is arrangedon the upper side of the carrier. This allows the sensor to be appliedin the desired location in a simple way.

[0035] When reading the sensor, a magnetic field M is applied, asalready mentioned, over the electric circuit 2, and the magnetic field Mwill be more or less affected as a function of the value of theconductivity of the substrate reaction products.

[0036] Advantageously, the sensor can be used to detect moisture in adiaper, for example an incontinence diaper. The sensor is applied to alabel in the diaper during manufacturing thereof. To check the diaperduring use the signal from the sensor is read by means of a detector inthe form of a mobile handheld tool that is applied in the vicinity ofthe diaper. A detector can also be mounted in the patient's bedforwarding any signals to a control station.

[0037] Another field of use for the sensor is detecting the moistureexposure of a moisture-absorbing bulk material. One example thereof maybe a fibre-based packaging material. The sensor registers the moistureexposure and when the packaging material is supplied to a packagingmachine a device reads the sensor to establish the moisture exposure.The settings of the packaging machine will then be dependent on, interalia, this moisture exposure.

[0038] The sensor may also be used to measure the moisture to whichgoods, such as packaging material and products, are exposed duringtransport and during storage. The label comprising the sensor is appliedto the goods when the products are packaged and is then checked by meansof a detector when the goods have reached their final destination.

[0039] The sensor may further be used to check the status in a dryingprocess and to detect liquids in hidden spaces such as leakage frompipes.

[0040] It will be understood that the present invention is not limitedto the embodiments shown. Several modifications and variations arepossible within the scope of the invention. The scope of the inventionis thus defined only by the appended claims.

1. A sensor for indicating moisture, comprising a unit (1) theconductivity of which is moisture exposure dependent and an electriccircuit (2), said unit (1) being included as a component in saidelectric circuit (2), and said electric circuit (2) being activable byapplying an electric field and/or a magnetic field (M) over the same togenerate a measurable signal, which is dependent on the total resistanceof the circuit (2), characterised in that said unit (1) comprises anenzyme (1 a) together with a substrate (1 b), which enzyme (1 a) isactivated by moisture exposure to catalyse a reaction with the substrate(1 b) that is dependent on the degree of moisture exposure, thesubstrate reaction product increasing the conductivity of the unit (1).2. A sensor according to claim 1, wherein the reaction product increasesthe conductivity of the unit (1) as a function of the time andtemperature at which moisture exposure occurs.
 3. A sensor according toclaim 1 or 2, wherein the enzyme (1 a) is selected from one of thefollowing enzyme classes: Transferases, Hydrolases and Lyases.
 4. Asensor according to claim 1, 2 or 3, wherein the enzyme (1 a) is Urease.5. A sensor according to any one of the preceding claims, wherein thesubstrate (1 b) comprises Urea.
 6. A sensor according to any one of thepreceding claims, wherein the unit (1) is made of an absorbent material.7. A sensor according to claim 6, wherein the absorbent material isfilter paper.
 8. A sensor according to any one of the preceding claims,wherein the unit (1) comprises a layer that allows moisture transportfrom the surroundings.
 9. A sensor according to claim 8, wherein thelayer prevents evaporation from the absorbent material.
 10. A sensoraccording to any one of the preceding claims, wherein the electriccircuit (2), except said unit (1), is insulated against moisture.
 11. Asensor according to any one of the preceding claims, wherein said signalis a measurable current, which is generated in the circuit (2).
 12. Asensor according to any one of the preceding claims, wherein the circuit(2) is an oscillation circuit.
 13. Use of a sensor according to claims1-12 for indicating moisture in a diaper.
 14. Use of a sensor accordingto claims 1-12 for indicating moisture in transported goods such aspackaging material or products.
 15. Use of a sensor for indicatingmoisture, the sensor comprising: a unit (1), comprising an enzyme (1 a)and a substrate (1 b), which enzyme (1 a) when activated catalyses areaction with the substrate (1 b), the reaction product of whichincreases the conductivity of the unit (1), and an electric circuit (2),said unit (1) being included as a component in said electric circuit(2), and said electric circuit (2) being activable by applying anelectric field and/or a magnetic field (M) over the same to generate ameasurable signal, which is dependent on the total resistance of thecircuit (2), characterised in that said unit (1) is exposed to thesurroundings and said enzyme (1 a) is adapted to be activated only whenexposed to moisture from the surroundings.
 16. A method of indicatingmoisture by means of a sensor, in which (i) the sensor is arranged on orin a product or product container, (ii) an enzyme (1 a) in the sensor isactivated by moisture, (iii) the degradation of a substrate (1 b) iscatalysed by the enzyme (1 a), (iv) an electric field and/or a magneticfield is applied over an electric circuit (2) included in the sensor,(v) a signal generated by the circuit (2) is measured, (vi) the currentmoisture of the product or product container is indicated on the basisof said signal.